1. Field of the Invention
The present invention relates to polymeric foam useful for acoustical attenuation.
2. Description of Related Art
There is a desire to increase efficiency in terms of cost and time of producing thick foam useful for acoustical attenuation (acoustical foam) production. Time and cost tends to increase dramatically as the thickness of the foam increases because production becomes more complicated. The primary difficulty with increasing the thickness of an acoustical foam is in the step of perforating the foam.
Acoustical foams are often perforated to facilitate blowing agent dissipation and to reduce airflow resistivity. Blowing agent dissipation is desirable to exchange blowing agent that may be flammable or otherwise undesirable with air. See, for example, U.S. Pat. No. 5,585,058. Acoustical foams also presumably require a substantially open-cell structure and a relative low airflow resistivity to be acoustically active (see, for example, U.S. Pat. No. 6,720,362 at column 1, lines 41-44 and column 10, lines 29-31, cited portions incorporated herein by reference).
Perforating foam becomes increasingly difficult as foam thickness increases. Rollers containing spikes can be suitable for perforating thin foams by rolling the spikes over the foam and impressing the spikes into the foam. Such a technique becomes problematic with thicker foams if perforations are to achieve appreciable depth. Perforating an appreciable distance into thick foams requires relatively long spikes or needles. Rolling long spikes into a foam tends to tear the foam as the spikes enter and exit the foam. Therefore, perforating thick foams typically requires impaling the foam in a single direction onto a bed of needles (or needles into the foam) and then drawing the needles out from the foam in the same direction. It is difficult to incorporate such a perforation procedure into a continuous process so efficiency decreases in regards to time of manufacturing. Moreover, the cost of equipment for impaling foam with a bed of needles tends to be as much as ten times that of a roller containing spikes. Therefore, efficiency decreases from a cost perspective as well.
Blowing agent dissipation also becomes more problematic as foam thickness increases. Perforation channels, through which blowing agent travels to escape from cells internal to a foam, become longer and more tortuous as foam thickness increases. Gas takes longer to permeate through a longer more tortuous channel than a shorter less tortuous channel. Thicker foams require longer perforation channels to reach internal cells. As a result, the longer the perforation channel, the longer it takes for the blowing agent to find its way out of the foam. Hence, even when perforated, thicker foams tend to suffer from slower blowing agent dissipation than thinner foams that are perforated. The slower the dissipation of blowing agent, the longer the foam must be stored before selling. As a result, slow blowing agent dissipation is undesirably costly in time and money.
Despite drawbacks to preparing thicker foams, increasing the thickness of foam is desirable. Increasing foam thickness tends to increase the acoustical dampening ability of the foam, particularly in low frequency ranges.
It is desirable to be able to increase the thickness of acoustical foam without having to experience the difficulties in perforating the foam and dissipating the blowing agent typically associated with thicker acoustical foams while maintaining or improving acoustical activity of the polymeric foam.